Water Separation Under Reduced Pressure

ABSTRACT

Water is separated from a liquid mixture (e.g., sea water) using a humidification chamber and a dehumidification chamber that are each operated at a pressure less than ambient atmospheric pressure (e.g., at least 10% less than ambient atmospheric pressure). A carrier gas is flowed through the humidification chamber; and inside the humidification chamber, the carrier gas directly contacts the liquid mixture to humidify the carrier gas with water evaporated from the liquid mixture to produce a humidified gas flow. The humidified gas flow is directed through the dehumidification chamber, where water is condensed from the humidified gas flow and collected.

BACKGROUND

Desalination of seawater or brackish water is generally performed byeither of the following two main processes: (a) by evaporation of watervapor or (b) by use of a semi-permeable membrane to separate fresh waterfrom a concentrate. In a phase-change or thermal processes, thedistillation of seawater is achieved by utilizing a heat source. In themembrane processes, electricity is used either for driving high-pressurepumps or for establishing electric fields to separate the ions.

Important commercial desalination processes based on thermal energy aremultistage flash (MSF), multiple-effect distillation (MED) and thermalvapor compression (TVC). The MSF and MED processes consist of manyserial stages at successively decreasing temperature and pressure.

The multistage flash process is based on the generation of vapor fromseawater or brine due to a sudden pressure reduction (flashing) whenseawater enters an evacuated chamber. The process is repeatedstage-by-stage at successively decreasing pressures. Condensation ofvapor is accomplished by regenerative heating of the feed water. Thisprocess requires an external steam supply, normally at a temperaturearound 100° C. The maximum operating temperature is limited by scalingformation, and thus the thermodynamic performance of the process is alsolimited.

For the multiple-effect distillation system, water vapor is generated byheating the seawater at a given pressure in each of a series ofcascading chambers. The steam generated in one stage, or “effect,” isused to heat the brine in the next stage, which is at a lower pressure.The thermal performance of these systems is proportional to the numberof stages, with capital cost limiting the number of stages to be used.

In thermal vapor compression systems, after water vapor is generatedfrom the saline solution, the water vapor is thermally compressed usinga high pressure steam supply and then condensed to generate potablewater.

A second important class of industrial desalination processes usesmembrane technologies, principally reverse osmosis (RO) andelectrodialysis (ED). Reverse osmosis employs power to drive a pump thatincreases the pressure of the feed water to the desired value. Therequired pressure depends on the salt concentration of the feed. Thepumps are normally electrically driven. For reverse osmosis systems,which are currently the most economical desalination systems, the costof water production can go up to US$3/m³ for plants of smaller capacity(e.g., 5 to 100 m³/day). Also, reverse osmosis plants require expertlabor for operation and maintenance purposes. The electrodialysisprocess also requires electricity to produce migration of ions throughsuitable ion-exchange membranes. Both reverse osmosis andelectrodialysis are useful for brackish water desalination; reverseosmosis, however, is also competitive with multi-stage flashdistillation processes for large-scale seawater desalination.

The multistage flash process represents more than 75% of the thermaldesalination processes, while the reverse osmosis process representsmore than 90% of membrane processes for water production. Multistageflash plants typically have capacities ranging from 100,000 to almost1,000,000 m³/day. The largest reverse osmosis plant currently inoperation is the Ashkelon plant, at 330,000 m³/day.

Other approaches to desalination include processes such as theion-exchange process, liquid-liquid extraction, and the gas hydrateprocess. Most of these approaches are not widely used except when thereis a requirement to produce high purity (total dissolved solids<10 ppm)water for specialized applications.

Another interesting process that has garnered much attention recently isthe forward osmosis process. In this process, a carrier solution is usedto create a higher osmotic pressure than that of seawater. As a resultthe water in seawater flows through the membrane to the carrier solutionby osmosis. This water is then separated from the diluted carriersolution to produce pure water and a concentrated solution that is sentback to the osmosis cell. This technology is yet to be provencommercially.

The technology that is at the root of this invention is known as thehumidification-dehumidification (HDH). The HDH process involves theevaporation of water from a heated water source (e.g., sea water) in ahumidifier, where the evaporated water humidifies a carrier gas. Thehumidified carrier gas is then passed to a dehumidifier, where the wateris condensed out of the carrier gas.

The predecessor of the HDH cycle is the simple solar still. In the solarstill, water contained in an enclosure is heated by sunlight to causeevaporation, and the evaporated water is condensed on a glass cover ofthe enclosure and collected. The most prohibitive drawback of a solarstill is its low efficiency (generally producing a gained-output-ratioless than 0.5). The low efficiency of the solar still is primarily theresult of the immediate loss of the latent heat of condensation throughthe glass cover of the still. Some designs recover and reuse the heat ofcondensation, increasing the efficiency of the still. These designs(called multi-effect stills) achieve some increase in the efficiency ofthe still, but the overall performance is still relatively low. The maindrawback of the solar still is that the various functional processes(solar absorption, evaporation, condensation, and heat recovery) alloccur within a single component.

SUMMARY

The apparatus and methods, described herein, include improvements uponthe humidification-dehumidification process for separating water.Various embodiments of this invention, as characterized in the claims,may include some or all of the apparatus, methods, elements, featuresand steps described below.

A method for separating water from a liquid mixture that includes wateruses a humidification chamber and a dehumidification chamber that areeach operated at a pressure less than ambient atmospheric pressure. Inthe method, a carrier gas is flowed through the humidification chamber.Inside the humidification chamber, the carrier gas directly contacts theliquid mixture to humidify the carrier gas with evaporated water fromthe liquid mixture to produce a humidified gas flow. The humidified gasflow is then directed through the dehumidification chamber, where wateris condensed from the humidified gas flow and collected. The totalpressure inside both the humidification chamber and the dehumidificationchamber is less than ambient atmospheric pressure (for example, at least10% less than ambient atmospheric pressure—in particular embodiments,about 90 kPa or less).

The humidifier and the dehumidifier are substantially thermallyseparated from each other (i.e., there is no more than minimal heattransfer via direct thermal conduction between the chambers of each;thermal energy is instead primarily transferred between the chambers viamass flow of the liquid and gas). The carrier gas can be recycledthrough the apparatus in a closed loop, and the carrier gas and/or theliquid mixture is heated in the apparatus, e.g., by solar energy, wasteheat, or fossil fuel. When the gas flow is heated, it can be heatedafter it leaves the humidification chamber and before it enters thedehumidification chamber. All processes in the apparatus can be poweredby renewable energy sources and/or by waste heat; accordingly, somerealizations of the apparatus can be operated off-grid (i.e., withoutany electrical connection to distributed electrical power grid) and inremote areas with few resources. The liquid mixture can be, for example,in the form of sea water, brackish water or ground water.

Desalination technologies based on the apparatus and methods of thisdisclosure are advantageous in that they have modest energy requirementsfor operation and can utilize renewable energy (e.g., solar energy,which is the most abundantly available energy resource on the earth, orgeothermal or wind energy) or waste heat energy from other processes inremote (off-grid) areas to power the efficient production ofsubstantially pure water. In contrast, conventional processes, such asmultistage flash and reverse osmosis require large amounts of energy inthe form of thermal energy (for multistage flash) or electric power (forreverse osmosis) and typically are fossil-fuel driven. The use of fossilfuels in conventional approaches results in a large carbon footprint forthe desalination plant, and sensitivity to the price and availability ofoil. Moreover, owing to their fossil fuel dependence, conventionaldesalination techniques are less applicable for decentralized waterproduction. Decentralized water production is important for regions thatlack the infrastructure and the economic resources to run multistageflash or reverse osmosis plants and that are sufficiently distant fromlarge scale production facilities to render pipeline distributionprohibitive. Many such regions are found in the developing world inregions of high incidence of solar radiation.

Additional advantages that can be achieved with the apparatus andmethods, described herein, include (a) an improved gained-output-ratio;(b) low cost for the apparatus and its operation and maintenance; (c) areduced production of brine disposal due to a lower recovery ratio incomparison with other desalination systems; and (d) excellentsuitability for small-scale installations (e.g., producing from 1 to 100m³ of substantially pure water per day) in, for example, remote desertlocations where resources are scarce, though sunlight and salt water areavailable.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a humidification-dehumidificationcycle and apparatus that utilizes liquid mixture heating.

FIG. 2 is an illustration of a humidification-dehumidification cycle andapparatus that utilizes liquid mixture heating.

FIG. 3 is an illustration of a multi-extractionhumidification-dehumidification cycle and apparatus that utilizes liquidmixture heating.

FIG. 4 is a schematic illustration of a humidification-dehumidificationcycle and apparatus that utilizes carrier gas heating.

FIG. 5 is a plot showing the effect of atmospheric pressure on theperformance (gained output ratio) for one embodiment of ahumidification-dehumidification cycle that utilizes carrier gas heating.

FIG. 6 is an illustration of a humidification-dehumidification cycle andapparatus that utilizes carrier gas heating.

FIG. 7 is an illustration of a multi-extractionhumidification-dehumidification cycle and apparatus that utilizescarrier gas heating.

In the accompanying drawings, like reference characters refer to thesame or similar parts throughout the different views. The drawings arenot necessarily to scale, emphasis instead being placed uponillustrating particular principles, discussed below.

DETAILED DESCRIPTION

The foregoing and other features and advantages of various aspects ofthe invention(s) will be apparent from the following, more-particulardescription of various concepts and specific embodiments within thebroader bounds of the invention(s). Various aspects of the subjectmatter introduced above and discussed in greater detail below may beimplemented in any of numerous ways, as the subject matter is notlimited to any particular manner of implementation. Examples of specificimplementations and applications are provided primarily for illustrativepurposes.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a,” “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. Additionally, the terms, “includes,” “including,” “comprises”and “comprising,” specify the presence of the stated elements or stepsbut do not preclude the presence or addition of one or more otherelements or steps.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this invention belongs. It will befurther understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art andwill not be interpreted in an idealized or overly formal sense unlessexpressly so defined herein. For example, if a particular composition isreferenced, practical, imperfect realities may apply; e.g., thepotential presence of at least trace impurities (e.g., at less than 0.1%by weight or volume) can be understood as being within the scope of thedescription.

Although the terms, first, second, third, etc., may be used herein todescribe various elements, these elements should not be limited by theseterms. These terms are only used to distinguish one element fromanother. Thus, a first element, discussed below, could be termed asecond element without departing from the teachings of the exemplaryembodiments.

In this disclosure, when an element is referred to as being, forexample, “on,” “connected to” or “coupled to” another element, it may bedirectly on, connected or coupled to the other element or interveningelements may be present. Additionally, spatially relative terms, such as“above,” “upper,” “beneath,” “below,” “lower,” and the like, may be usedherein for ease of description to describe the relationship of oneelement to another element, as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the apparatus in use or operation in additionto the orientation depicted in the figures.

As used herein, gained-output-ratio (GOR) is the ratio of the latentheat of evaporation of the distillate produced to the energy input(e.g., net heat absorbed by one or more solar collectors or the net heatinput to the system by other means). The GOR represents the energyefficiency of water production and is an index of the amount of the heatrecovery effected in the system.

The humidification-dehumidification (HDH) cycle involves thehumidification of a carrier gas by a liquid mixture that contains waterfollowed by the dehumidification of the humidified carrier gas torelease pure water.

The separation of the humidification and dehumidification functions intodistinct components in a humidification-dehumidification apparatus canreduce thermal inefficiencies and improve overall performance. Forexample, recovery of the latent heat of condensation in thehumidification-dehumidification process is affected in a separate heatexchanger (i.e., the dehumidifier) in which the seawater, for example,can be preheated. Additionally, the module for solar collection can beoptimized almost independently of the humidification or condensationcomponent. The humidification-dehumidification process thus can providehigher productivity due to the separation of the basic processes.

Using the apparatus, described below, the principle ofhumidification-dehumidification of a carrying gas is utilized toseparate water from a liquid mixture. The liquid mixture can be in theform of a solution with dissolved components (such as salts) and/or amixture containing solids and/or other liquids. The process is hereindescribed in the context, for example, of water desalination, where purewater is separated from salt water, though the process and apparatus canlikewise be utilized in the context of separating water from otherliquid mixtures.

In a humidification-dehumidification cycle that utilizes liquid mixtureheating, as is schematically shown in FIG. 1, a carrying gas (such asair) is circulated through gas conduits 16 and 18 between a humidifier12 and a dehumidifier 14 in, e.g., a closed loop system. The humidifier12 and dehumidifier 14 are of a modular construction (i.e., separateparts) and are substantially thermally separated from one another. Thecharacterization of the humidifier and dehumidifier as being“substantially thermally separated” is to be understood as beingstructured for little or no direct conductive heat transfer through theapparatus between the humidification and dehumidification chambers,though this characterization does not preclude the transfer of thermalenergy via gas and/or liquid flow between the chambers. This“substantial thermal separation” characterization thereby distinguishesthe apparatus from, e.g., a dewvaporation apparatus, which includes ashared heat-transfer wall between the humidifier and the dehumidifier.In the apparatus of this disclosure, the humidifier and dehumidifier donot share any common walls that would facilitate conductive heattransfer therebetween.

Instead, thermal energy is transferred between the chambers mostly viamass flow of the gas and liquid. The gas is humidified in thehumidification chamber 20 of the humidifier 12 using the hot impurewater (i.e., the liquid mixture—for example, in the form of an aqueoussaline solution), which is sprayed from one or more nozzles 22 at thetop of the humidifier 12 while the gas moves in a counter flow direction(up through the humidification chamber 20, as shown), thereby increasingthe water vapor content in the gas via evaporation of water from theliquid mixture into the carrier gas flow. The remaining portion of theliquid mixture that is not evaporated in the humidification chamber 20,pools at the bottom of the chamber 20 and drains through aliquid-mixture output conduit 30.

The humidified carrier gas is then directed through conduit 16 to thedehumidifier 14, where the carrier gas is dehumidified in adehumidification chamber 24 using the cold inlet liquid mixture pumpedthrough a liquid-mixture input conduit 26 and through a coiled conduit28 inside the dehumidification chamber 24, allowing for heat transferfrom the gas to the liquid mixture inside the dehumidifier 14. The watervapor in the gas therefore condenses and is collected as substantiallypure water at the bottom of the dehumidification chamber 24. Thecollected pure water can then be removed from the dehumidifier 14through pure-water output conduit 32 for use, e.g., as drinking water,for watering crops, for washing/cleaning, for cooking, etc. The carriedgas can be circulated between the humidifier and dehumidifier naturallyor by using a fan. If a fan is used for gas circulation, the fan may bepowered by a photovoltaic solar panel or by a wind turbine, and the fanmay be put in the top gas conduit or in the bottom gas conduit.

After being preliminarily heated in the dehumidifier 14, the liquidmixture is passed via liquid-mixture conduit 34 to the humidifier 12. Aheater 36 can be included in or along the conduit 34 to further heat theliquid mixture before entering the humidifier. The heater 36 may use asolar energy source (e.g., the heater may be in the form of a solarcollector) and/or may use any waste heat source (e.g., use waste heatgenerated by other nearby machinery or by a power generating apparatus)to heat the liquid mixture.

In this process, the pressure inside both the humidifier 12 and thedehumidifier 14 is reduced below the ambient atmospheric pressure (i.e.,lower than about 101 kPa at sea level), in contrast with previoushumidification-dehumidification desalination processes that work underambient atmospheric pressure. As the pressure inside the humidifier 12decreases, the ability of the humidified gas to carry more water vaporincreases, thereby providing increased production of the pure water whenthe gas is dehumidified in the dehumidifier 14. This can be explained bythe humidity ratio (i.e., the ratio of water vapor mass to dry air massin moist air), as it is higher at pressures lower than atmosphericpressure. For example, air (as a carrier gas) at a dry bulb temperatureof 60° C., the saturation humidity ratio at 50 kPa is roughly 150%higher than at atmospheric pressure.

Our analysis shows that by reducing the operating pressure inside thehumidification chamber 20 and inside the dehumidification chamber 24 toa pressure of, for example, 50 kPa, the gained output ratio (GOR) of theprocess increases. The GOR increases from about 2.4 at standardatmospheric pressure (about 101 kPa) to about 2.55 at about 35 kPa(which is a 6.25% increase).

A practical and simple solution for the system to be working under loweratmospheric pressure without any significant additional energy input isillustrated in FIG. 2, where a static head is fixed between the cycleequipment (including the humidifier 12 and the dehumidifier 14) and theliquid-mixture tank 42 and the pure-water tank 44. The distance 58between the liquid/water levels 46 and 48 in the humidifier 12 anddehumidifier 14, respectively, to the liquid/water levels 50 and 52 atthe tanks' surface can be, for example, between 15 feet (4.6 meters) and20 feet (6.1 meters) to produce a Torricellian vacuum inside thechambers 20 and 24. Increasing the distance from 15 feet to 20 feet willreduce the pressure inside the cycle from 50 kPa to 40 kPa. Thesub-ambient-atmospheric pressure in both the humidification chamber 20and dehumidification chamber 24 can be substantially the same and canbe, for example, at least 10% less than ambient atmospheric pressure,e.g., 90 kPa or less; or, in particular embodiments, 70 kPa or less; or,in more-particular embodiments, between 10 and 60 kPa.

As shown in FIG. 2, the humidification chamber 20 can be filled with apacking material 56 in the form, e.g., of polyvinyl chloride (PVC)packing to facilitate turbulent gas flow and enhanced direct contactbetween the carrier gas and the liquid mixture. The body of thehumidifier (and the dehumidifier) can be formed, e.g., of stainlesssteel and is substantially vapor impermeable; seals formed, e.g., ofepoxy sealant, gaskets, O-rings, welding or similar techniques, areprovided at the vapor and water inputs and outputs of the humidifier andat the interfaces of each modular component and adjoining conduits tomaintain vacuum in the system. In one embodiment, humidification chamber20 is substantially cylindrical with a height of about 1.5 m and aradius of about 0.25 m.

As can be pictured via the image of FIG. 2, humidification of thecarrier gas is achieved by spraying the liquid mixture from one or morenozzles 22 into a spray zone at the top of the humidifier 12 thenthrough a packing material 56 and down through a rain zone to a surface46 of collected liquid mixture at the bottom of the chamber, while thecarrier gas moves up through the humidification chamber 20, as shown,and is brought into contact with the liquid mixture, particularly in thebed of packing material 56, to add water vapor from the liquid mixtureto the carrier gas.

Additionally, the remaining liquid mixture in the water tank 42 can bepumped via a water pump 54, which can be powered by a photovoltaic solarpanel or by a wind turbine, through conduit 26 so that it can be pumpedback through the system to evaporate more water from the liquid mixturein the humidifier 12. The water tank 42 may be connected to a large bodyof the liquid mixture (e.g., sea, ocean, groundwater, etc.) in which thewater concentration of the liquid mixture does not change withevaporation in the humidifier. Otherwise, water concentration in thetank 42 can be monitored, and blow-down and make up can be provided tokeep the water concentration in tank 42 within an operating limit.

FIG. 3 shows the same humidification-dehumidification cycle thatutilizes liquid mixture heating with a multi-extraction configuration,wherein the gas is extracted from a plurality of distinct intermediatelocations in the humidifier 12 and fed to corresponding distinctintermediate locations in the dehumidifier 14 via gas conduits 60, 62and 64, allowing for manipulation of gas mass flows, thermal balancingof equipment and for a higher recovery of heat. Alternatively, gas canflow in the opposite direction through conduits 60, 62 and 64 from thedehumidifier 14 to the humidifier. The gas can flow through the conduits60, 62 and 64 naturally, or the flow can be powered by a fan in one ormore of the conduits. The amount of extracted gas depends strongly onthe operating conditions, and this amount can be controlled via variablespeed fans (if fans are used) or by adjusting the extraction conduitsize (i.e., diameter and/or length).

Instead of directly heating the liquid mixture, as shown in FIGS. 1, 2and 3, the circulated carrier gas can be heated by a gas heater 66 ingas conduit 16 after the carrier gas passes through the humidifier 12,as shown in the gas-heated humidification-dehumidification cycle of FIG.4. The gas heater 66 can be, e.g., a solar air heater. Heating thehumidified carrier gas in turns heats the liquid mixture in thedehumidification chamber 24. The performance of this cycle, measured asgained output ratio (GOR) for one exemplary embodiment of the apparatusand method, is shown in FIG. 5 for an embodiment with an inlet feedwater temperature of 30° C., a heated air temperature of 67° C., and ahumidifier and dehumidifier effectiveness of 90%. As shown in FIG. 5,The GOR in this embodiment ranges from about 3.5 at ambient atmosphericpressure to about 4 at about 50 kPa and then to about 4.5 at 30 kPa(which is a 28.5% increase in GOR). A detailed drawing of the gas-heatedcycle is shown in FIG. 6, while a detailed drawing of themulti-extracted configuration of the gas-heated cycle is shown in FIG.7, where the gas is extracted from multiple intermediate locations inthe dehumidification chamber 24 to be fed to corresponding intermediatelocations in the humidification chamber 20, through conduits 60, 62 and64 allowing for equipment thermal balancing and a higher recovery ofheat. Providing the multi-extracted configuration for the gas heatedcycle (FIG. 7) can increase the GOR to higher values. Again, the gas mayflow in either direction through conduits 60, 62 and 64.

In describing embodiments of the invention, specific terminology is usedfor the sake of clarity. For purposes of description, each specific termis intended to at least include all technical and functional equivalentsthat operate in a similar manner to accomplish a similar result.Additionally, in some instances where a particular embodiment of theinvention includes a plurality of system elements or method steps, thoseelements or steps may be replaced with a single element or step;likewise, in some instances, a single element or step may be replacedwith a plurality of elements or steps that serve the same purpose.Further, where parameters for various properties are specified hereinfor embodiments of the invention, those parameters can be adjusted up ordown by 1/100^(th), 1/50^(th), 1/20^(th), 1/10^(th), ⅕^(th), ⅓^(rd), ½,¾^(th), etc. (or up by a factor of 2, 5, 10, etc.), or by rounded-offapproximations thereof, unless otherwise specified. Moreover, while thisinvention has been shown and described with references to particularembodiments thereof, those skilled in the art will understand thatvarious substitutions and alterations in form and details may be madetherein without departing from the scope of the invention. Furtherstill, other aspects, functions and advantages are also within the scopeof the invention; and all embodiments of the invention need notnecessarily achieve all of the advantages or possess all of thecharacteristics described above. Additionally, steps, elements andfeatures discussed herein in connection with one embodiment can likewisebe used in conjunction with other embodiments. The contents of allreferences, including reference texts, journal articles, patents, patentapplications, etc., cited throughout the text are hereby incorporated byreference in their entirety. Appropriate components and methods of thosereferences may be selected for the invention and embodiments thereof.Still further, the components and methods identified in the Backgroundsection can be used in conjunction with or substituted for componentsand methods described elsewhere in the disclosure within the scope ofthe invention. In method claims, where stages are recited in aparticular order—with or without sequenced prefacing characters addedfor ease of reference—the stages are not to be interpreted as beingtemporally limited to the order in which they are recited unlessotherwise specified or implied by the terms and phrasing.

1. A method for substantially separating water from a liquid mixtureincluding water, the method comprising: directing a flow of a carriergas through a humidification chamber in at least one humidifier;directly contacting the flow of carrier gas with the liquid mixture inthe humidification chamber to humidify the carrier gas with waterevaporated from the liquid mixture, producing a humidified gas flow,wherein the total pressure in the humidification chamber is less thanambient atmospheric pressure; directing the humidified gas flow througha dehumidification chamber in at least one dehumidifier; dehumidifyingthe humidified gas flow in the dehumidification chamber to condensewater from the humidified gas flow, wherein the total pressure in thedehumidification chamber is less than the ambient atmospheric pressure;and collecting the condensed water.
 2. The method of claim 1, furthercomprising collecting the non-evaporated content of the liquid mixturein a bottom portion of the humidification chamber as water is evaporatedfrom the liquid mixture, wherein the water released in thedehumidification chamber is collected in a bottom portion of thedehumidification chamber.
 3. The method of claim 2, wherein awater-filled receptacle is positioned beneath the dehumidificationchamber, and wherein a water-filled conduit extending from the bottomportion of the dehumidification chamber into the water-filled receptaclemaintains the sub-atmospheric pressure in the dehumidification chamber,and wherein a liquid-mixture-filled receptacle is positioned beneath thehumidification chamber, and wherein a liquid-mixture-filled conduitextending from the bottom portion of the humidification chamber into theliquid-mixture-filled receptacle maintains the sub-atmospheric pressurein the humidification chamber.
 4. The method of claim 1, wherein thecarrier gas is directed through the humidifier and the dehumidifier in aclosed loop.
 5. The method of claim 1, further comprising heating theliquid mixture before it enters the humidification chamber.
 6. Themethod of claim 5, wherein the liquid mixture is heated via solarenergy, waste heat, or fossil fuel.
 7. The method of claim 1, whereinthe gas flow is extracted from at least one intermediate location in thehumidification chamber or in the dehumidification chamber and fed fromeach extracted intermediate location to a corresponding intermediatelocation in the dehumidification chamber or in the humidificationchamber, respectively, allowing for manipulation of gas mass flows andfor greater heat recovery
 8. The method of claim 1, further comprisingheating the humidified gas flow before it enters the dehumidificationchamber.
 9. The method of claim 8, wherein the humidified gas flow isheated via solar energy, waste heat, or fossil fuel.
 10. The method ofclaim 1, wherein the pressure in the humidification chamber and in thedehumidification chamber is 90 kPa or less.
 11. The method of claim 1,wherein the pressure in the humidification chamber and in thedehumidification chamber is 70 kPa or less.
 12. The method of claim 1,wherein the pressure in the humidification chamber and in thedehumidification chamber is between 10 and 60 kPa.
 13. The method ofclaim 1, further comprising circulating the carrier gas through thehumidification chamber and through the dehumidification chambernaturally or by a fan powered by an energy source selected from at leastone of the following: solar energy and wind energy.
 14. The method ofclaim 1, further comprising circulating the liquid mixture through thehumidifier and through the dehumidifier by a pump powered by an energysource selected from at least one of the following: solar energy andwind energy.
 15. The method of claim 1, wherein the apparatus isoperated free of any electrical coupling with an electrical power grid.16. The method of claim 1, wherein the liquid mixture is selected fromseawater, brackish water or groundwater.
 17. The method of claim 1,wherein the dehumidifier is substantially thermally separated from thehumidifier in a modular construction.
 18. A method for desalinatingwater, the method comprising: directing a flow of a carrier gas througha humidification chamber in at least one humidifier; directly contactingthe carrier gas flow with an aqueous saline solution in thehumidification chamber to humidify the carrier gas with water evaporatedfrom the saline solution, producing a humidified gas flow; collectingthe non-evaporated content of the saline solution in a bottom portion ofthe humidification chamber and directing the collected non-evaporatedcontent of the saline solution down through a conduit to asaline-solution-filled receptacle below the humidification chamber tomaintain a sub-atmospheric pressure in the humidification chamber to atleast 10% less than ambient atmospheric pressure; directing thehumidified gas flow through a dehumidification chamber in at least onedehumidifier, wherein the dehumidifier is substantially thermallyisolated from the humidifier; dehumidifying the humidified gas flow inthe dehumidification chamber to condense water from the humidified gasflow; extracting the gas flow from at least one intermediate location inthe humidification chamber or in the dehumidification chamber andfeeding the gas flow from each extracted intermediate location to acorresponding intermediate location in the dehumidification chamber orin the humidification chamber, respectively, allowing for manipulationof gas mass flows and for greater heat recovery; and collecting thecondensed water in a bottom portion of dehumidification chamber anddirecting the collected water down through a conduit to a water-filledreceptacle below the dehumidification chamber to maintain asub-atmospheric pressure in the dehumidification chamber that is atleast 10% less than ambient atmospheric pressure.